This is a randomized, double-blind clincal trial involving subjects with prolonged prior experience with ZDV of ddI monotherapy. Eligible subjects are participants in ACTG 175 whi remained on their initial assigned monotherapy until the end of follow-up in ACTG 175 (data collection closed of Feb.28,1995) and for any intervening period until entry into ACTG 302. This clinical trial comprises two sub-groups. One is for subjects who were assigned ZDV monotherapy in ACTG 175; these subjects will be randomized to receive eithe d4T monotherapy or the combination ot 3TC and ZDV, in a blinded manner. The other is for subjects who were assigned ddI monotherapy in ACTG 175; these subjects will be randomized to receive eithe the combination of ddI and ZDV or the combination of 3TC and ZDV, in a blinded manner. The primary outcome measure is change in HIV RNA copy number. The duration of study treatment is planned to be 48 weeks.